It says right on it that it's attorney work product of the Department of Justice, which is what one would expect when asking how the FBI interprets a legal decision. The FBI would have asked their lawyers, who happen to be the DOJ, how the law should be interpreted. The advice they received would be privileged as attorney/client advice, and therefore not subject to release under FOIA. It's pretty disingenuous of the ACLU (of which I'm a fan, normally) to pretend otherwise.

My goodness - the comments section is a myriad of stupidity. Actually in all of the contents only 2 posts actually are bringing up the fact that legal interpretation would be covered by attorney client privilege - the kind of stuff that a FOI would deny. It takes a court order to compel such communication and I can't imagine a lawyer fighting tooth and nail to oppose such a ruling if it ever came up (which I doubt has happened) and they might even face contempt since they would refuse such an order.